1. Field of the Invention
This invention relates to a bone-replacement prosthetic device, and in particular, to a method and apparatus for segmental bone replacement.
2. Description of the Related Art
In prosthetic segmental bone replacements, it is sometimes necessary to replace a portion of a long bone to correct various types of bone injury such as those caused by bone tumors, osteoarthritis, fracture dislocations, rheumatic arthritis, and aseptic or avascular bone necrosis. In these types of surgical procedures, it is necessary to resect a mid and/or end portion of a long bone and secure the remaining portion of bone through the use of some type of intramedullary device. This is accomplished by the use of one metal stem, or in certain cases, two opposing metal stems, secured in the medullary region by a xe2x80x9cbone cementxe2x80x9d or a grout material, such as methylmethacrylate.
Biologic fixation of intramedullary devices at the mid-diaphyseal level has not been entirely satisfactory, particularly in active younger patients, where it is important to form a stable, long-lasting prosthetic attachment. In time, the lack of adequate stress transfer from the metal stem to the surrounding bone causes a loss of bone density, resulting in increased possibility of bone failure or loosening of the bone-stem interface. Also, the bone reacts in the grout material or smooth metal stem by forming a soft-tissue lining around the cement, and this lining additionally mediates load transfer from the prosthetic device to the bone. The soft-tissue lining that forms about the device tends to loosen over time, particularly with continued shear loads, i.e., loads applied substantially in the direction of the axially extending bone/stem interface, and the loosening may become great enough in time to require surgical revision. Also, the relatively low tolerance of force transfer per unit area of interface requires a large bone/stem interface, which, in younger patients, may exceed the available interface area.
In replacement surgeries, which begin at the mid-diaphyseal level, the enlarged intramedullary cavities created within a remaining diaphyseal bone portion for the insertion and rigid fixation of a metal intramedullary device stem is also of small diameter. As a result, the device stems must be of a small diameter to fit within the diaphysis, with resulting poor rotational control. Also, high bending moments in thin stems at the mid-diaphysis risk fatigue failure, making a stable interface between the stem and the surrounding bone surface even more difficult to achieve.
Although the use of a tapered stem is one way to achieve increased security of the stem within the medullary canal, the canal dimensions tend to increase rather than decrease with increasing depth into the canal, thereby preventing secure wedging of a tapered stem. Consequently, xe2x80x9cbone cementxe2x80x9d or grout fixation is often used to secure the stem within the medullary canal. This technique, however, prevents stress transfer through the bone to the level of the osteotomy, therefore resulting in osteopenia adjacent to the stem. The same effect prevents bony ingrowth into porous pads on the shoulders of the implants. A limitation of prosthetic devices which rely on biological fixation, particularly fixation to an elongate stem within the intramedullary region of a bone, is the problem of stress protection of the bone region between the area of force application to the prosthesis and the area of load transfer to the bone. Stress protection is due to the rigid attachment between the prosthetic device and bone which occurs in biological fixation and to the relatively high elastic modulus of the implant material, which typically is five to fifteen times greater than that of the surrounding bone. These two factors combine to transfer a stress from the area of stress loading on the implant through the more rigid implant, rather than through the surrounding bone tissue. For example, in a hip-joint prosthesis biologically anchored to the bone by an entire elongate stem, axial stress on the upper joint is transferred largely through the stem to the bone connection farthest from the joint, rather than through the intermediate bone region surrounding the part of the stem closest to the joint. As a result, the intermediate bone region tends to be resorbed over time due to lack of deformation stressing. The gradual loss of bone support in the region of the stem increases the bending load that must be borne by the stem, and this can lead to implant fatigue and failure.
The problem of maintaining a motionless bone-prosthesis interface during the post-operative period when bony attachment is occurring may be partially solved by surgically fastening the prosthetic device to the bone structure by screws or the like. This method has been proposed for use in fastening a knee-joint prosthesis to a surgically formed, substantially planar surface of the bone. Typically, the prosthesis is attached by two or more screws, each tightened to hold the prosthesis against the bone surface with a selected compression. However, since the bone quickly accommodates to the applied force of the screws, by viscoelastic creep, the compression, and thus the resistance to the implant movement relative to the bone, is quickly lost. If interface movement does occur from a single episode of overloading, then any residual compression is permanently lost. More movements result in build-up of fibrous tissue, preempting biological bone fixation to the implant. Only with unphysiologic post-operative protection of the joint, resulting in joint stiffness and muscle wasting, and with demanding operative technique, can risk of loosening be reduced. The device also suffers from problems of stress protection and non-physiological load transfer, inasmuch as loading force applied to the prosthesis is transferred directly through the screws, rather than through the region of bone through which the screws extend. This can lead to loss of bone integrity in the stress protected area.
The problems associated with anchorage via soft tissue along a prosthesis stem have been overcome partially by using a prosthesis whose stem surface allows direct attachment without an interposed soft tissue layer. Such surfaces include micropore surfaces that allow attachment via ingrowth and/or attachment of bone, and ceramic surfaces that allow actual bonding of bone. Following surgical implantation of the stem, the surrounding bone tissue gradually forms a biological fixation matrix with the stem surface by tissue growth into or onto the surface. Because of the stronger interface between the bone and the stem, which allows a relatively large force per unit area without loosening, problems of late loosening and detachment are largely avoided and the force transfer area can be made smaller.
A limitation of the biological-fixation bonding approach, however, is the need to keep the prosthesis mechanically fixed with respect to the bone over a 2-3 month post-operative period, during which the biological fixation is occurring. If relative movement between the implant stem and bone is allowed to occur before biological fixation is complete, a fibrous tissue layer which acts to prevent good biological fixation develops at the interface and eventual progression to gross loosening is likely.
Another shortcoming resulting from bone replacement surgeries, particularly joint replacement procedures, is a phenomenon known as wear particle bone lysis. The replacement of a joint, including the installation of a polyethylene or similar high molecular weight synthetic wear surface, results over time in particles becoming dislodged from the wear surface due to friction between the joint sections during movement. These wear particles tend to move with fluid transfer along the interface between the prosthesis and the surrounding soft tissue, and also tend to enter the intramedullary space between the prosthesis stem and the surrounding remaining bone portion. The biological reaction to these small wear particles causes the surrounding bone tissue to be lysed, thereby weakening the bone and potentially causing subsequent bone failure. Thus, a means for sealing the intramedullary space from the exterior space could largely reduce this difficulty.
Prosthetic devices having spring-loaded mechanisms for holding a joint-replacement prosthesis against a planar surface of the bone, to immobilize the prosthesis on the bone, have been proposed, e.g., in the related field of joint replacements, such as in U.S. Pat. No. 4,129,903. Devices of this type solve some of the above-noted problems associated with prosthesis attachment to the bone, in that the prosthesis is held against the bone under relatively constant tension in the post-operative period, with or without provision for biological fixation. Nonetheless, limited movement may occur when the major loading stresses (in the principal direction of weight transfer on the joint) are not normal to the plane of the interface between the bone and prosthetic device and it is necessary to rely on a grouting compound to prevent shear motions. Further, such devices use a rigid stem or shaft for anchoring the implant to the bone traversed by the stem from physiologic shear, rocking, and/or axial rotation stresses.
Each of these potential problems may limit physical activity and long-term durability prognosis for long segmental replacement arthroplasty patients. Thus, current methods may potentially result in repeat surgeries which leave less bone stock and may eventually require amputation or other undesirable salvage procedures.
For the above reasons, it is desirable to provide a segmental bone replacement device which enhances a stable biologic fixation, yet allows for physiologic cyclic load transfer to the device-bone interface. It is also desirable to provide a device which promotes osteogenesis into those surfaces adjacent to the osteotomy.
The present invention provides a biocompatible bone attachment assembly which is a connector or segment replacing a section of a long bone diaphysis, such as the remaining portion of a resected femur. The assembly includes at least a biocompatible bone attachment device which is secured to a first remaining bone portion and may be connected to an opposing orthopedic appliance or a transcutaneous bar. The attachment device and the orthopedic appliance or transcutaneous bar may be secured in an adjacent relation by a suitable securing means.
Compliant fixation is a significant advance in this field because it provides compliance of fixation force which maintains compression of the implanted device at the device-bone interface while allowing axial compression or elongation of the remaining bone portion. In addition, compliant fixation provides excursion in that it allows compensation for loosening of the implant device following installation to accommodate settling of the implant device against the bone. In the use of compliant fixation devices, the implant is constructed to be more compliant than the surrounding bone, that is, the spring constant of the implant is by design less than the spring constant of the bone with regard to any or all of bending, torsion and axial compression.
A bone attachment device of the present invention includes a main body having an interface surface for abutting against an interface surface of a remaining bone portion. The device further includes means for anchoring the bone attachment device within an enlarged cylindrical intramedullary cavity of the remaining bone portion. This is preferably provided as an anchor body which is secured within the enlarged intramedullary cavity through one or more transverse pins or interlocking screws which engage both the anchor body and the surrounding bone.
The bone attachment device further includes compliant means for attaching the main body to the means for anchoring the bone attachment device. This preferably includes a compliant connecting rod which extends from the anchor body through the osteotomy surface.
The means for attaching the main body to the means for anchoring the bone attachment device may also include a supplemental means for biasing the connecting rod against the main body. This is preferably provided as a supplemental interposed compliant device disposed in communication with both the connecting rod and the main body, which is most preferably one or more, washer springs secured upon the connecting rod against a surface or recess of the main body by a retaining means.
The device may further include one or more means for enhancing a fluid seal of the intramedullary cavity from the external environment, which may be provided as one or more rings or similar sealing members disposed at one or more suitable locations upon or within the main body.
The present invention may further include a reaming device for creating an enlarged cylindrical intramedullary cavity within the first remaining bone portion. The reaming device is operated by connection to a drilling device, such as a hand drill.
The present invention may further include a guide device for guiding an aperture-forming procedure upon the remaining bone portion. The guide device is operable to be assembled onto a protruding portion of an integrated connecting rod and anchor body above the osteotomy surface.
The present invention may further include a milling device for shaping the osteotomy surface in a preselected geometry. The milling device is also operated by connection to a hand drill.
In the method of the present invention, a bone is resected to yield at least a first remaining bone portion. A cylindrical intramedullary cavity is then created to a preselected depth, through the use of a reaming device as described herein, which cavity is suitable for receiving the bone attachment device of the present invention.
Means for anchoring the bone attachment device, such as an anchor body, or an integrated connecting rod and anchor body, followed by means for attaching the main body to the means for anchoring the bone attachment device, such as a connecting rod, is inserted into the cavity. A guide device, as described herein, is attached to the inserted connecting rod at its protrusion from the osteotomy surface, and is adjusted to guide an aperture-forming procedure upon the first remaining bone portion. Thereafter, the anchor body is engaged against the surrounding bone by the insertion of one or more engagement devices, such as transverse pins or interlocking screws, through the apertures previously created with the assistance of the guide device. The osteotomy surface of the first remaining bone portion is milled in a preselected geometry for promoting bone ingrowth, through the use of a milling device as described herein. The main body is then slipped into position on the end of the bone. Anti-rotation pins are drilled and inserted. One or more supplemental interposed compliant devices, which is preferably one or more washer springs, may then be positioned upon the connecting rod, and enhanced in a secured relation upon the connecting rod through the use of one or more devices, such as a nut and a lock nut. One or more fluid seal means may also be positioned upon the opening between the connecting rod and the main body, within a recess of the main body, or upon the exterior of the main body.
The above steps may be repeated with respect to an opposing remaining portion of the bone, for connection to the bone attachment device through an interposed orthopedic appliance. A means for enhancing a secured relation, such as a sleeve clamp, may then be positioned about the main body and an opposing member which is an opposing orthopedic appliance or a transcutaneous bar.
The present invention also provides a bone attachment assembly which may be a connector or segment replacing a section of bone, such as a long bone diaphysis or a proximal femur, or may also be a primary bone replacement or a knee or other joint replacement. The assembly includes a main body having an interface surface for abutting against an abutment surface of a first remaining bone portion. The assembly also includes means for anchoring the bone attachment assembly in a substantially stationary position within a cavity located within the first remaining bone portion. This cavity may be a natural intramedullary canal, an enlarged intramedullary canal or a cavity created through the bone at any suitable location. The assembly further includes a compliant section that is preferably integrally formed between the main body and the means for anchoring the bone attachment assembly. The compliant section is operable for being converted to a preselected condition of expansion. The present invention also includes a method for implanting the bone attachment assembly.
The present invention also provides a sleeve disposed upon a compliant section of bone attachment assembly. This sleeve is operable for inhibiting deflection of the compliant section in a non-axial direction during expansion or contraction of the compliant section associated with physiologic loading. The sleeve may preferably be integrally formed with the main body of the device. The present invention also discloses different configurations for the main body and an anchor body that is connected to the main body. Specifically, the anchor body may be secured across the surrounding bone cortex through pins or screws or may alternately include a cylindrical or tapered, self-tapping threaded section for being threaded directly into the bone. The present invention also discloses a compliant section that is integrally formed with the anchor body. Connection of the anchor body and compliant section to another component such as a tibial tray is also contemplated. The compliant section is expanded by force exerted by the anchor body in a direction away from the main body and force applied against the main body in an opposite direction by various traction-applying means, such as a rod integrally formed with the compliant section or threadably secured to the compliant section, and secured with respect to the main body. In addition, instruments are shown and described in accordance with the method of the present invention which involves reaming an intramedullary cavity of a remaining bone portion to an enlarged condition, parallel cross-pinning or threadably securing an anchor body and compliant section within the cavity, positioning the main body upon the anchor body and applying traction through various means to the compliant section.
Accordingly, it is a general object of the present invention to provide a method and apparatus for segmental bone replacement, for primary bone replacement or for a knee or other joint replacement. A related object of the present invention is to provide a method and apparatus for such bone replacement which enhances a biological and mechanical attachment of a bone attachment assembly to a remaining bone portion.
A further object of the present invention is to provide a method and apparatus for segmental bone replacement which allows for physiologic cyclic load transfer to the apparatus-bone interface.
An additional object of the present invention is to provide a method and apparatus for segmental bone replacement which promotes osteogenesis into those surfaces adjacent to the osteotomy.
A further object of the present invention is to provide a method and apparatus for reducing access of wear particles from joint replacements from entering the intramedullary space of a bone following resection.
An additional object of the present invention is to reduce resorption of bone adjacent to an installed bone attachment device.
An additional object of the present invention is to introduce a precision complex technique in such a way that it can be adapted to routine surgical practice.
Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.